Lithography
Generally, lithography is used to transfer a specific pattern onto a surface. Lithography can be applied to transfer a variety of patterns including, for example, painting, printing, and the like. More recently, lithographic techniques have become widespread for use in “microfabrication”—a major (but non-limiting) example of which is the manufacture of integrated circuits such as computer chips.
In a typical microfabrication operation, lithography is used to define patterns for miniature electrical circuits. Lithography defines a pattern specifying the location of metal, insulators, doped regions, and other features of a circuit printed on a silicon wafer or other substrate. The resulting circuit can perform any of a number of different functions. For example, an entire computer can be placed on a chip.
A primary lithography system includes a wafer stepper, a photomask and photoresist. A wafer stepper generally consists of a ultraviolet (UV) light source, a photomask holder, an optical system for projecting and demagnifying the image of the mask onto a photoresist-coated wafer, and a stage to move the wafer. Conventional lithography also generally requires a photomask—a quartz substrate with chrome patterns on one surface. The chrome patterns form a perfect master of the pattern to be inscribed on one layer of a chip. Also it requires photoresist to receive the light pattern generated by the mask.
Improvements in lithography have been mainly responsible for the explosive growth of computers in particular and the semiconductor industry in general. The major improvements in lithography are mainly a result of a decrease in the minimum feature size (improvement in resolution). This improvement allows for an increase in the number of transistors on a single chip (and in the speed at which these transistors can operate). For example, the computer circuitry that would have filled an entire room in 1960's technology can now be placed on a silicon “die” the size of a thumbnail. A device the size of a wristwatch can contain more computing power than the largest computers of several decades ago.
The resolution of a photolithography system is described by the Rayleigh equation:d=k1λ/NAwhere d is the minimum feature size, λ is the wavelength, NA is the numerical aperture of the optical system and k1 is a constant determined by a specific system. For a certain wavelength and a certain optical design, the only way to improve the resolution is to increase the numerical aperture. The numerical aperture is defined as:NA=n sin θwhere n is the refraction index of the relative medium and θ is the half angle of the cone of rays received by the entrance pupil. High NA indicates high light collecting or light focusing power. It is rather straightforward to see that the resolution is proportional the refractive index of the medium.Semiconductor Nano-Sized Particles
Nano-sized particles are loosely defined as powders with small diameters for example ranging from a few hundred nanometers or less down to a few angstroms. Since they have generally only been the focus of research in the last two decades, there is little standardization, and they take many different names including quantum dot, quantum sphere, quantum crystallite, nano-crystal, micro-crystal, colloidal particle, nano-cluster, Q-particle or artificial atom. Due to their small size, they often possess dramatically different physical properties compared to their bulk counterparts. Nano-sized particles have a wide range of applications including metallurgy, chemical sensors, pharmaceuticals, painting, and cosmetics. As a result of the rapid development in synthesis methods in the last two decades, they have now entered into microelectronic and optical applications. Nano-sized particles with sizes less than 5 nm have been synthesized from a variety of semiconductors, examples include C, Si, Ge, CuCl, CuBr, CuI, AgCl, AgBr, AgI, Ag2S, CaO, MgO, ZnO, ZnS, HgS, ZnSe, CdS, CdSe, CdTe, HgTe, PbS, BN, AlN, GaN, GaP GaAs, GaSb, InP, InAs, InxGa1-xAs, SiC, Si1-xGex, Si3N4, ZrN, CaF2, YF3, Al2O3, SiO2, TiO2, Cu2O, Zr2O3, SnO2, YSi2, GaInP2, Cd3P2, Fe2S, Cu2S, CuIn2S2, MoS2, In2S3, Bi2S3, CuIn2Se2, In2Se3, HgI2, PbI2, Lanthanoids oixides, etc. They have revealed very interesting optical properties.
Semiconductor materials have the so called bandgaps. The electron band below the bandgap is call valence band (VB) and the electron band above the bandgap is called conduction band (CB). The manifestation of a bandgap in optical absorption is that only photons with energy larger than the bandgap are absorbed. A photon with sufficient energy excites an electron from the top of valence band to the bottom of conduction band, leaving an empty state, a hole, at the top of the valence band.
There are several major advantages of using semiconductor nano-sized particles in photolithography. First, the bandgap of semiconductor nano-sized particles can be tailored by their size. In a certain range the smaller the size, the larger the bandgap. The bandgap determines the working wavelength.
Second, the refractive index can be very high near the bandgap. Actually some semiconductors have the highest refractive indices. For example wurzite TiO2 has a refractive index of 2.4, and wurzite GaN has a refractive index about 2.6 near the bandgap. The refractive indices of common optical materials such as fused silica and quartz used in the UV lithography are around 1.5. This high refractive index is desirable for highly refractive medium immersion lithography and optical coating.
Third, nano-sized particles can be easily coated onto optics or wafers in the form of a thin film. They are, therefore, very simple to handle and produce much less contamination. Because of the polycrystalline nature of nano-sized particle films, there is less concern about matching the thermal expansion coefficients between the coating and the optics. Applying nano-sized particles by coating provides least disturbance to the existing lithography system.
Fourth, semiconductors nano-sized particles can reach sizes much smaller than the working wavelength. Currently, a large number of semiconductors can be fabricated into nano-sized particles smaller than 5 nm in diameter. Hence the scattering from the nanoparticles is negligible and size fluctuation of nano-sized particles does not affect the final scattered and transmitted light.
Fifth, in a broad sense semiconductors can possess bandgaps as high as 12 eV, corresponding to a wavelength of 100 nm. For 157 nm lithography and beyond, few materials can withstand the radiation except certain semiconductors. Nano-sized particles offer a solution for the optics in these wavelengths.
Lastly, many semiconductor nano-sized particles can be produced rather inexpensively. Therefore, the overall cost will likely be lower than conventional methods.
We propose several applications of semiconductor nano-sized particles in lithography. Such as highly refractive medium in immersion lithography, optical coating, pellicle material, and sensitizer in photoresists.